1. Field of the Invention
Embodiments of this present invention relate to a direct heat exchange method and apparatus for recovering heat from a liquid heat source.
Embodiments of this present invention relate to a direct heat exchange method and apparatus for recovering heat from a liquid heat source, where the method includes contacting a liquid heat source stream with a multi-component hydrocarbon fluid, where the hydrocarbon fluid compositions has a linear or substantially linear temperature versus enthalpy relationship over the temperature range of the direct heat exchange apparatus.
2. Description of the Related Art
Highly mineralized geothermal brine, which is cooled in a heat recovery process will, at some given temperature, start to release or precipitate solid minerals (mostly silica). The precipitating solid materials settle out on surfaces of heat transfer equipment drastically reducing the heat transfer coefficient of the equipment. After some time, the contaminated heat exchange equipment will become completely blocked and unworkable.
In prior art, this problem was addressed using a direct contact heat exchange apparatus. In these prior art direct contact heat exchange apparatuses, an immiscible liquid heat transfer fluid is was brought into direct contact in a counter-flow relationship with the liquid heat source. The heat transfer fluid is heated as the heat source liquid is cooled.
Because the heat source liquid and the heat transfer fluid in this arrangement both have practically constant specific heat values, the heat recovery in this process is thermodynamically very efficient. Moreover, the heat transfer coefficients in such direct contact heat exchange apparatuses are higher than in conventional heat exchange apparatus.
In this approach, as the heat transfer fluid and the heat source liquid move in counter-flow, the only driving force for this movement is a difference in specific gravities of the fluids.
For purposes of heat recovery from geothermal brines, oils or liquid hydrocarbons are usually used as the heat recovery liquid.
Because the difference in the specific gravities of the heat transfer fluid and the heat source fluid is usually quite small, the velocity with which both liquids move in the direct heat exchange apparatus has to be quite low to avoid flooding.
An alternate approach in the prior art was used by the Barber-Nichols Company, where an Organic Rankine Cycle (ORC) working fluid (usually isobutane or isopentane) was vaporized in direct contact with a geothermal brine in a counter-flow relationship. However, a single component working fluid boils at a constant temperature, whereas the heat released by the geothermal brine is released at variable temperatures or over a temperature range. Therefore, this prior art approach, while useful for the vaporization of a single component ORC working fluid, is not efficient for heat recovery in cases where the heat exchange fluid is then used to transfer heat to an alternate working fluid of a power cycle (or, for that matter, to any other fluid for other applications.)
Thus, there is a need in the art for a novel method and apparatus for direct heat exchange from a heat source to a fluid designed to have a change enthalpy that is linear with the change in temperature along the length of an active heat exchange zone.